There are various drying methods, such as sun drying, boiling drying, drying, spray drying and vacuum drying, etc., but ordinary drying methods are usually carried out at temperatures above 0 ℃ or higher. The dried products generally have the problems of shrinking volume and hardening texture. Most of the volatile components will be lost. Some heat-sensitive substances are denatured and deactivated, and some substances even oxidize. Therefore, compared with the product before drying, there is a big difference in the characteristics of the product after drying. The lyophilization rule is basically carried out below 0 ℃, that is, the product is frozen, and the analytical drying generally does not exceed 60 ℃. Under vacuum, when the water vapor directly sublimates out, the drug remains in the ice shelf when it freezes, forming a sponge-like loose porous structure, so its volume is almost unchanged after drying. Before use again, as long as water for injection is added, it will immediately dissolve.
　　Compared with the conventional method, the freeze-drying method has the following advantages:
　　* Many heat-sensitive substances will not be denatured or inactivated.
　　* When drying at low temperature, some volatile components in the material lose little.
　　* During the freeze-drying process, the growth of microorganisms and the action of enzymes cannot be carried out, so the original properties can be maintained.
　　* Because it is dried in a frozen state, the volume is almost unchanged, the original structure is maintained, and the concentration phenomenon does not occur.
　　* Since the water in the material exists in the form of ice crystals after pre-freezing, the dissolved substances of inorganic salts originally dissolved in water are evenly distributed in the material. During sublimation, the dissolved substances dissolved in water are precipitated, which avoids the surface hardening caused by the precipitation of inorganic salts carried by the internal moisture of the material to the surface in the general drying method.
　　* The dried material is loose and porous, showing a sponge-like shape. After adding water, it dissolves quickly and completely, and the original properties are restored almost immediately.
　　* Because the drying is performed under vacuum, there is very little oxygen, so some easily oxidized substances are protected.
　　* Drying can exclude more than 95% to 99% of water, so that the dried product can be stored for a long time without deterioration.
　　* Because the material is in a frozen state and the temperature is very low, the temperature of the heat source for heating is not high, and a heater with normal temperature or low temperature can meet the requirements. If the freezing room and the drying room are separated, the drying room does not need to be insulated, and there will not be a lot of heat loss, so the use of heat energy is very economical.
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